1. Field of the Invention
The Present invention relates to a motor driving circuit, and more Particularly to a current controlled type motor driving circuit in which the starting characteristics of a speed control circuit are improved by the control of negative impedance.
2. Description of the Related Art
A conventional motor driving circuit of the kind to which the present invention relates is, as shown in FIG. 1, one in which the motor speed Is controlled by a current controlled method using negative impedance. As shown in FIG. 1, in the conventional motor driving circuit, a power supply source Vcc is connected to one input terminal of the motor 10 to be controlled and an output terminal 11 of a control circuit 13 is connected the other input terminal thereof; the power supply source Vcc is also connected to one end of a resistor R; the other end of the resistor R is connected to one end of a variable resistor R.sub.1 and also to a reference voltage input terminal 12 of the control circuit 13; and the other end of the variable resistor R.sub.1 is connected to the output terminal 11 of the control circuit 13 and also the other input terminal of the motor 10.
The potential difference across the two input terminals of the motor 10 is controlled so as to be made equal to a voltage which is the sum of a voltage drop V.sub.R developed across the register R and a reference voltage produced by a reference voltage generating circuit provided in the control circuit 13. The voltage drop V.sub.R can be accounted for by the product of a resistance R and a current which is the sum of the current I.sub.1 flowing into the control circuit 13 and a current I.sub.2 of a value resulting from the division of the reference voltage produced by the reference voltage generating circuit provided in the control circuit 13 by the resistor R.sub.1. That is, the control circuit 13 detects changes in a counterelectromotive force of the motor due to changes in the rotation number caused by the load changes on the motor 10 and compares the detected changes with the reference voltages. The difference thus obtained causes the value of the current flowing to the reference voltage input terminal 12 to be changed and the voltage drop V.sub.R to be changed. Thus, the potential difference across the two terminals of the motor 10 changes correspondingly with the load changes and the number of revolutions of the motor 10 is kept constant.
If the load on the motor 10 increases, the current I.sub.M which flows to the motor increases and also the current I.sub.1 which flows to the control circuit 13 correspondingly increases. This results in an increase in the voltage drop V.sub.R at the resistor R as the potential difference across the two terminals of the motor 10 increases. Thus, the motor 10 can attain a rotation torque that is sufficiently strong to overcome the increase in the load torque and can rotate steadily.
Now, the details of a conventional motor driving circuit are explained. FIG. 2 is a circuit diagram showing a conventional motor driving circuit of a type in which negative impedance is used to control a current which in turn controls the speed of a motor. The conventional motor driving circuit shown in FIG. 2 is constituted by a control circuit 23 blocked in dashed lines, a resistor R and a variable resistor R.sub.1. The control circuit 23 is constituted by a constant-current circuit 24, a reference voltage generating circuit 25, a comparator circuit 26, an output driving circuit 27, and a symmetric current circuit 28. The symmetric current circuit 28 is constituted by a transistor Q.sub.1 and a transistor Q.sub.2.
The reference voltage produced by the reference voltage generating circuit 25 and the voltage drop across the resistor R connected to the reference voltage input terminal 22 are summed together to give a reference voltage potential Va. The comparator circuit 26 compares this potential Va with the output terminal potential Vb which is a voltage at the output terminal 21 of the control circuit 23. The output of the comparator circuit 26 is inputted to and amplified at the output driving circuit 27. The output of the output driving circuit 27 is then inputted to the symmetric current circuit 28. The symmetric current circuit 28 is formed by a current mirror circuit constituted by the transistor Q.sub.1 and Q.sub.2.
First, the operation of the motor under normal conditions, that is, in the state in which the motor rotation is under control, is explained. At the comparator circuit 26, the reference voltage potential Va and the output terminal potential Vb are compared and the obtained result is inputted to one of the input terminals of the motor 20 through the output driving circuit 27 and the symmetric current circuit 28 and is also inputted to the comparator circuit 27 as the output terminal potential Vb. Thus, the comparator circuit 26, the output driving circuit 27 and the symmetric current circuit 28 constitute a closed loop circuit, which maintains a constant potential difference across the two input terminals of the motor 20 and keeps the rotation of the motor 20 constant.
FIGS. 3A, 3B and 3C are explanatory diagrams showing the relationship between potentials at various points in the motor driving circuit shown in FIG. 2. FIG. 3A shows the relationship between potentials at various points in the control circuit 23 when the load on the motor 20 is light. In this condition, the symmetric current circuit 28 causes the current Ic proportional to the current I.sub.M flowing to the motor to flow to the resistor R through the reference voltage input terminal 22 of the control circuit 23, thereby balancing the reference voltage potential Va and the output terminal potential Vb which are the potentials at the two input terminals of the comparator circuit 26. In FIGS. 3A, 3B and 3C, Ra.I.sub.M indicates a voltage drop due to the internal resistance Ra of the motor 20; Ea indicates the counterelectromotive force of the motor 20; Ic.R is the voltage drop at the resistor R when the current in the resistor R is set to a value equivalent to the current in the internal resistor Ra of the motor 20; (Icc+Is+I.sub.2)R indicates a voltage drop across the resistor R due to the currents which flow from the reference voltage input terminal 22 to the constant-current circuit 24, the voltage reference circuit 25 and the variable resistor R1; and Vr indicates a reference voltage outputted from the reference voltage generating circuit 25.
FIG. 3B shows the relationship between potentials at various points in the control circuit 23 when the load on the motor 20 is heavy. In this situation, although the current I.sub.M flowing to the motor increases with an increase in the voltage drop Ra.I.sub.M across the internal resistance Ra of the motor 20, the resistor R is proportional to the internal resistance Ra of the motor 20 and the voltage drop across the resistor R also increases. Consequently, as the load on the motor 20 becomes heavier, the control circuit 23 detects the current flowing to the motor thereby increasing the voltage drop across the resistor R and also increasing the potential difference across the two terminals of the motor 20. In this way, the rotation of the motor 20 is kept constant irrespective of the magnitude of the load on the motor.
FIG. 3C shows the relationship between potentials at various points in the control circuit 23 at the starting of the motor 20. Generally, in a DC motor such as the motor 20, the counterelectromotive force gently increases from zero during the starting of the motor so that, at the starting moment, the counterelectromotive force and current I.sub.M are zero with the transistor Q.sub.2 being OFF and the output terminal 21 outputting a voltage substantially equal to the power source voltage Vcc. The voltage produced at the output terminal 21 becomes the output terminal potential Vb which is an input voltage applied to one input terminal of the comparator circuit 26. On the other hand, the reference voltage Va which is an input voltage applied to the other input terminal of the comparator circuit 26 becomes lower than the input terminal potential Vb due to the function of the reference voltage generating circuit 25.
As explained above, during the starting of the motor 20, since the reference voltage potential Va becomes {Vcc-(V.sub.R +Vr)} with the output terminal potential Vb being about the same as the power source voltage Vcc, the difference between the reference voltage potential Va and the output terminal potential Vb becomes large. Due to such an increase in the potential differences between the reference voltage potential Va and the output terminal potential Vb, the transistor Q.sub.2 in the symmetric current circuit 28 becomes conductive and, thus, the motor 20 starts rotating due to the current which flows from the collector to the emitter of the transistor Q.sub.2.
On the other hand, during the maximum driving state of the transistor Q.sub.2, the potential V.sub.1 at the reference voltage input terminal 22 drops to V.sub.1min which is determined by the circuit configuration of the control circuit 23. Consequently, since the current I.sub.M flowing to the motor upon starting is proportional to a portion of the current flowing to the control circuit 23, the maximum value I.sub.1max of the total current I.sub.1 flowing through the resistor R and the maximum value Ic.sub.max of the current Ic flowing to the collector of the transistor Q.sub.1 may thus be expressed by the following Equations 1 and 2, respectively. EQU I.sub.1max =(Vcc-V.sub.1min)/R=Icc+Ic.sub.max +Is (1) EQU Ic.sub.max =(Vcc-V.sub.1min)/R (2)
Here, since I.sub.1max &gt;&gt;I2, Ic.sub.max &gt;&gt;Icc+Is, and I.sub.M =K.Ic, the starting current of the motor may be expressed by the following Equation 3: EQU I.sub.Mmax =K. (Vcc-V.sub.1min)/R (3)
Here, the current Ic flowing to the collector of the transistor Q.sub.1 is of a value proportional to the current I.sub.M flowing to the motor which is the current flowing to the collector of the transistor Q.sub.2.
As described above and shown in FIG. 3C, during the starting of the motor 20, the reference voltage potential Va applied to the comparator circuit 26 becomes lower than the output terminal potential Vb also applied to the comparator circuit 26.
In the conventional motor driving circuit described above, one of the problems is that, since the starting current characteristics of the motor are determined by the configuration of the motor driving circuit, sufficient starting torque cannot be obtained during the starting operation of the motor. This is a problem to be solved by the invention, in the conventional motor driving circuit.